Mechanical devices that are used to control electrical current, such as relays, switches, contactors, and circuit breakers, rely upon electrical contacts to make and the break the flow of current. Electrical contacts usually include two conductive members which are arranged to engage and make contact in a particular way so that electricity flows across the junction between the engaged contacting surfaces of the contacts with relatively little loss. The contacting surfaces of electrical contacts are commonly engaged by such actions as impact, wiping, and/or rolling. Regardless of the particular type of mechanical action employed, electrical contacts are subject to several damage and failure mechanisms.
The making and breaking action of electrical contacts is often accompanied by plasma generation, or arcing, between the contacting surfaces of the contacts as they are opened and/or closed. Such arcing can damage the contacting surfaces by causing pitting, transfer of material, or promoting undesirable chemical reactions such as oxide formation. These arc-induced damage mechanisms can aggravate impact and/or frictional wear of the contacting surfaces due to the mechanical action of the contact. The useful life of an electrical contact, i.e. the number of switching operations or the length of time that the contact functions before exhibiting mechanical failure or unacceptably high electrical resistance, is limited by the deleterious effects of the various damage mechanisms upon the electrical characteristics of the contacts.
Electrical contacts are often made of a copper base. Copper is a relatively good electrical and thermal conductor and in low current applications, such as household current switching, acceptable contacts may be formed entirely of copper. However, for switching currents of higher magnitudes, such as the currents required by industrial motors, contacting surfaces of copper possess unacceptable properties. Copper contacting surfaces tend to form resistive oxide complexes which detract from switching performance. Additionally, copper contacting surfaces tend to weld together when used to conduct even moderately high current densities. Depending on severity, the welding together of contact surfaces can cause delayed turn-off response of relays or, in the worst case, can cause the contact connections to become permanently joined together making switching completely inoperable. Hence, several material compositions having acceptable physical properties have been formulated for use as contacting surface materials for electrical contacts. Silver alloys and solid suspensions have become popular compositions for use in contact surfaces of electrical contacts because silver has high electrical conductivity and high heat capacity. In order to produce contacting surfaces that are strong, resist wear, and have a reduced tendency to weld, silver is commonly used in conjunction with other metals, such as nickel, palladium, and tungsten. Silver is also used with metal oxides, such as oxides of cadmium and of tin and in chemical combination with other elements such as in silver carbide. Contacting surface compositions have been applied to copper contacts by riveting, welding, brazing, or sintering the selected composition onto the contacts in order to form contacting surfaces having shapes and sizes desired for various applications.
Still other techniques to extend contact life have focussed on the environment in which the contacts operate. In some switching devices, the contacts are located within sealed environments containing a vacuum or an inert atmosphere to reduce surface oxidation of the contacts. Sealed environments also prevent foreign material, such as dust, from accumulating upon or between electrical contacts and contributing to contact surface degradation. Various lubricants have also been used to reduce surface oxidation and to reduce wear. Despite such efforts, even highly specialized electrical contacts require periodic replacement or refurbishing due to deterioration of the contacting surfaces.
In accordance with the present invention, a method is provided that extends the useful life of electrical contacts in order to reduce the expense and inconvenience of replacement or repair of the contacts.